Dry atmospheric air consists of a mixture of nitrogen (78.1% by volume), oxygen (20.9%), inert gases (1.0%), and carbon dioxide (360 ppm, parts per million). The concentration of water vapor is seldom more than a few percent, but exhibits large geographical variations, depending on climate and weather. The CO2 concentration is both locally and globally of a dynamic character. Regardless of large buffer reservoirs of CO2 in the oceans, and in the earth's crust, the balance is determined by the build-up and breaking down of biological material. As a result of the combustion of fossil fuel, the CO2 concentration is increasing by approximately 2 ppm/year. A continuing increase is feared to cause significant climate changes, the so called green house effect. Locally, the outdoor CO2 concentration may rise a few hundred ppm above the mentioned background level, as a result of local biological processes, traffic intensity, industrial exhaust etc.
The indoor CO2 concentration is likewise an indicator of biological activity in relation to the ventilation of the locality with fresh outdoor air. Human expired air contains approximately 4% CO2, which results in fast increase of the indoor CO2 concentration at high person density and low ventilation rate. After entrance of a person in an empty room, an increasing CO2 concentration can be detected already after a few minutes. The concentration continues to rise until it reaches a level determined by the ventilation of the room. Excessively high CO2 concentration is associated with poor indoor environment, and 1000 ppm has been accepted as a hygienic upper limit by, among others, the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHORE). CO2 is thus used as a tracer gas or indicator of air quality. The concentration in ppm constitutes a quantitative measure which can be used for monitoring and control of indoor air quality. A kindred application area for CO2 sensors is personal presence detection, as a part of security systems. At fires the CO2 concentration increases rapidly already at the initial stage, which can be correspondingly put to use. Also worth mentioning are a number of medical applications. CO2 sensors can be used for respiratory patient monitoring, for evaluation of lung function, and respirator therapy control.
Instruments and transducers for the measurement of CO2 concentration in air have been described in the patent literature, and also in the form of commercially available products on the market. The majority of these are based on the absorption specter within the infrared wavelength area of electromagnetic radiation of the CO2 molecule. Such specters can be detected and analyzed by spectroscopic instruments according to known technology. By measuring at specific wavelengths where the absorption of CO2 deviates from other constituents of air, it is possible to extract an output signal with required sensitivity and specificity. One problem with this measuring principle is, however, contamination by dust, liquid drops, smoke particles etc, which put specific demands either on the design or on short maintenance intervals, in addition to high demands on mechanical precision. As a consequence, these instruments have a high production cost, but still limited reliability. They are therefore being put use only to a limited extent within the indicated application areas.